CI: use Docker Compose to run the container

Avoid redundant specification of Docker settings.

The compose file is set up to build all stages. This makes sense for
local development; both an interactive shell and running the webserver
are useful. Therefore, the image built is tagged "snekbox:dev".

However, CI does not need to run a webserver. It is therefore sufficient
for it to only build to the venv stage, and it does exactly that. The
image in CI is tagged as "snekbox-venv:<git sha>".

To facilitate the discrepancy in image tags, the suffix for the image
tag can be set with the new IMAGE_SUFFIX environment variable. Docker
Compose will use this to determine the image from which to create a
container.

A TTY needs to be allocated to prevent the container from exiting
immediately after starting. This is probably because the entrypoint
is Python (inherited from the base image), and the REPL relies on a TTY.

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